Plasma etching apparatus for processing wafers is known. Typically, a chuck serves as a lower electrode in a process chamber which can be set in a vacuum state. A wafer is placed on and fixed to the chuck, and then subjected to the plasma etching process. There are two commonly used ways of fixing a wafer to a chuck, mechanical supporting means such as a clamp, and an electrostatic chuck for attaching a wafer by means of an electrostatic attractive force. A typical electrostatic chuck includes a metallic base plate that is coated with a thick layer of slightly conductive dielectric material. A silicon wafer of approximately the same size as the chuck is placed on top of the chuck and a potential difference is applied between the silicon wafer and the base plate of the electrostatic chuck. This causes an electrostatic attraction proportional to the square of the electric field in the gap between the silicon wafer and the chuck face.
When the chuck is used in a plasma filled chamber, the electric potential of the wafer tends to be fixed by the effective potential of the plasma. The purpose of the dielectric layer on the chuck is to prevent the silicon wafer from coming into direct electrical contact with the metallic part of the chuck and shorting out the potential difference. On the other hand, a small amount of conductivity appears to be desirable in the dielectric coating so that much of its free surface between points of contact with the silicon wafer is maintained near the potential of the metallic base plate; otherwise, a much larger potential difference would be needed to produce a sufficiently large electric field in the vacuum gap between the wafer and chuck.
During plasma etching of pattern wafers, the plasma raises the temperature of the wafer to an undesirable level that could damage the wafer. Accordingly, the chuck must be kept as cool as possible. The current preferred method of cooling a plasma chuck is with conductive cooling of the backside of the chuck through the use of helium. The face of the chuck generally includes a pattern of grooves in which helium gas is maintained. This gas provides cooling by thermal contact between the wafer and the chuck. To contain the helium at the chuck and prevent it from escaping into the reaction, a clamp must be incorporated with the chuck to hold the wafer down.
In conventional pattern plasma etched apparatus, the chuck is stationary to allow for cooling. Non-uniform etching occurs, however, due to chamber design or process parameters resulting in undesirable film thickness deviations. These deviations in film thickness can be localized or spread across the entire film surface.